The present invention relates to a rolling bearing suitable for use under conditions where debris contamination and the temperature rise are large, and to an automotive gear shaft support device using the rolling bearing.
In an automotive differential or transmission, rolling bearings are used to support gear shafts. FIG. 1 shows an automotive differential in which a gear shaft is supported by a tapered roller bearing, which is one of the embodiments of the present invention. The differential comprises a drive pinion 4 rotatably supported in a housing 1 by two tapered roller bearings 2, 3, a ring gear 5 meshing with the drive pinion 1, a differential gear case 7 carrying the ring gear 5 and rotatably supported in the housing 1 by a pair of tapered roller bearings 6, pinions 8 mounted in the differential gear case 7, and a pair of side gears 9 meshing with the pinions 8. These are mounted in the housing 1 in which is sealed oil. The oil also serves as a lubricating oil for the tapered roller bearings 2, 3 and 6.
Since a power transmission device such as the above-described differential have many gear meshing portions and rotary member sliding portions, debris such as metallic powder produced at these portions tend to mix into oil sealed in the housing. Such powder may enter into rolling bearings which support gear shafts rotating at high speed and get caught into the raceways of rolling elements, thus causing surface peeling on the rolling elements or bearing rings. It is considered that such surface peeling is caused by initiation of cracks resulting from indentation due to debris.
In order to prevent initiation and progression of such cracks, in examined Japanese patent publication 62-29487, a technique is disclosed in which SUJ3-class steel is used as a bearing material, it is quenched at high temperature to increase the amount of retained austenite, and the cooling speed during quenching is slowed down to decrease crack sensitivity. In unexamined Japanese patent publication 7-190072, SUJ3-class steel is used for a bearing ring material, and carbo-nitriding is added to the above mentioned heat treatment to further increase the amount of retained austenite in the surface layer.
On the other hand, in recent use of roller bearings, in order to improve the rotation efficiency during high-speed rotation, there is a tendency to use a low-viscosity lubricating oil. Thus temperature rise at bearing portions tends to be large with an increase in metal-to-metal contact. In such use, because rolling elements are small in heat capacity and diffusion of heat due to contact with other members is small, their temperature rise is the largest. Thus, surface peeling tends to occur at the surface of rolling elements. Even if the material disclosed in Japanese patent publication 62-29487 is used, fatigue life is insufficient.
Further, with a tapered roller bearing, since each tapered roller rolls with its large end face guided by the large rib surface of the inner ring, a thrust load is loaded on the large rib surface of the inner ring. If the material disclosed in Japanese patent publication 7-190072 is used for the inner ring, although the rolling contact fatigue life under debris contamination may improve, the fatigue life of the rib portion bearing thrust load decreases.
An object of this invention is to provide a rolling bearing and an automotive gear shaft support device which can ensure a long endurance life even under use conditions where debris tends to enter and temperature rise is large.
According to this invention, there is provided a rolling bearing wherein rolling elements are made from a steel containing C: 0.8-1.5 wt %, Si: 0.4-1.2 wt %, Mn: 0.8-1.5 wt % and Cr: 0.8-1.8 wt %, wherein the steel is subjected to carbo-nitriding, and then quenched and tempered so that the amount of retained austenite in the surface portion is 20-50 vol %.
As for the composition of the steel as the material for the rolling elements, the carbon content should be 0.8 to 1.5 wt % to obtain basic hardness by quenching and also to increase the retained austenite.
The silicon (Si) content should be not less than 0.4 wt % to stabilize the retained austenite in the surface layer and to prevent softening at high temperature by adding Si, which has resistance to temper-softening. The upper limit is set at 1.2 wt % because if over 1.2 wt %, diffusion of carbon and nitrigen is prevented during carbo-nitriding.
The Mn content should be 0.8 to 1.5 wt % to increase hardenability and increase the amount of retained austenite in the surface layer. Excessive Mn content will result in lowering of cold workability and quenching crack. Also, the amount of retained austenite in the surface layer will be too large, so that quenching hardness tends to be low. Thus the upper limit is restricted to 1.5 wt %.
The Cr content should be 0.8 to 1.8 wt % because if less than 0.8 wt %, carbides would not be sufficiently formed even by carbo-nitriding, so that the hardness tends to decrease during temperature rise. If over 1.8 wt %, carbides tend to grow coarse and become starting points of cracks due to stress concentration during rolling contact.
By using a steel having such a composition as the material for rollers and increasing the nitrogen content in the surface layer by carbo-nitriding, the Ms point (martensitic transformation starting temperature) in the surface layer becomes lower than in the inner portion. By quenching such a steel, the amount of retained austenite increases in the surface layer of the rollers. The amount of retained austenite in the surface layer can thus be increased to 20 vol % or over.
Retained austenite has high toughness and work hardening properties, and serves to prevent crack initiation and propagation. But it is unstable to heat. Nitrogen atoms that have penetrated into the surface layer during the carbo-nitriding will solid soluted in austenite and make the retained austenite after quenching stable to heat. Also, the surface layer, in which the Ms point has decreased, the martensitic transformation begins later than in the core, and the transformation amount is smaller than in the core, so that compressive residual stress is formed in the surface layer. Thus, it is also possible to increase the fatigue strength of the surface layer.
The amount of retained austenite in the surface layer should be 20 to 50 vol % to give the surface layer suitable toughness, and to relieve stress: concentration due to contact with debris. If it is less than 20 vol %, toughness is not sufficient. If over 50 vol %, hardness will decrease remarkably, thus inviting deterioration in the surface roughness due to plastic deformation.
If debris gets caught or if the surface temperature rise is large, crack tends to initiate at the surface or at the subsurface (within 0.1 mm from the surface). Thus it is possible to prolong the life by improving the material of the surface layer in the above-described manner.
As the heat treatment including carbo-nitriding, carbo-nitriding may be carried out in a high-temperature gas in which ammonia gas is added to a carburizing atmosphere, and then followed by quenching and tempering. To adjust the amount of retained austenite, sub-zero treatment may be combined in the heat treatment step.
By adding 0.3 wt % or less of molybdenum to the steel as the material for rolling elements, it is possible to improve toughness.
Also, by quenching at 830-880xc2x0 C. and adjusting the cooling ability H during quenching in the temperature range of from 300xc2x0 C. to 150 xc2x0 C. to 0.2 cmxe2x88x921 or less, it is possible to slow down the cooling speed near the Ms point and to decrease the cracks sensitivity to 3 or less. If the crack sensitivity is 3 or less, it is possible to effectively prevent cracks during use, or to prevent the crack propagation speed even if cracks should initiate. As a means for adjusting the cooling ability during quenching in the temperature range from 300xc2x0 C. to 150xc2x0 C. to 0.2 cmxe2x88x921 or less, quenching in an oil or salt bath heated to a predetermined temperature may be employed.
If the rolling elements are needle rollers, and the amount of retained austenite in the surface layer is 20 to 40 vol %, and the surface hardness is Hv 750 or more, it is possible to prevent surface peeling of needle rollers and to prolong their life, even though they tend to heat up because they are low in heat capacity and are often mounted on a roughly finished shaft without a bearing ring.
The upper limit of the amount of retained austenite is set at 40 vol % to obtain a surface hardness of Hv 750 or over. Also, the surface hardness should be Hv 750 or over to ensure sufficient surface hardness even if the needle rollers, which are subjected to severe temperature rise, soften.
If the rolling elements are tapered rollers, wherein, of the inner and outer rings of the rolling bearing, at least the inner ring is made from case-hardened steel containing 0.4 wt % or less of carbon, and wherein on the surface layers of the inner and outer rings, carbo-nitrided layers are formed, it is possible to ensure fatigue strength of the rib portions of inner ring. If the carbon content exceeds 0.4 wt %, the core hardness during quenching would be too high, so that toughness becomes inferior and fatigue strength lowers. For the outer ring, it will do if a carbo-nitrided layer can be formed. Thus, besides case-hardened steel as used for the inner ring, it is possible to employ various kinds of bearing steel.
If the amount of retained austenite in the surface layer of the tapered rollers and inner and outer rings is 20-40 vol %, it is possible to provide suitable toughness to the surface layer of each part, and to prevent an excessive increase in stress due to contact with debris. If the retained austenite is less than 20 vol %, toughness will be insufficient. If over 40 vol %, hardness will decrease remarkably, resulting in deterioration of surface roughness due to plastic deformation. On the large end faces of the tapered rollers and the large rib surface of the inner ring, which are in slide contact with each other, seizure may occur due to deterioration in surface roughness.
According to this invention, there is provided a vehicular gear shaft support device wherein a gear shaft is rotatably supported by a tapered roller bearing in a housing in which is sealed oil, characterized in that the tapered rollers are made from a steel containing C: 0.8-1.5 wt %, Si: 0.4-1.2 wt %, Mn: 0.8-1.5 wt % and Cr: 0.8-1.8 wt %, that at least the inner ring is made from a case-hardening steel containing 0.4 wt % or less of carbon, and that a carbo-nitrided layer is formed on the surface layers of the tapered rollers, and inner and outer rollers, it is possible to remarkably prolong the maintenance cycle of e.g. differentials and transmissions.
In the gear shaft support device, by determining the amount of retained austenite in the surface layers of the tapered rollers and inner and outer rings at 20-40 vol %, it is possible to provide suitable toughness to the surface layer of each part, and to prevent an excessive increase in stress due to contact with debris.
Other features and objects of the present invention will become apparent from the following description made with reference to the accompanying drawings, in which: